Browsing by Author "Link, Stephan"
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Item A Comparison of Plasmon-induced and Photoexcited Hot Carriers in Metallic Nanostructures(2015-12-02) Zhao, Hangqi; Halas, Naomi; Nordlander, Peter; Link, StephanThe incompressible oscillations of electrons in metallic nanostructures, known as surface plasmons, have provided a promising route to increasing light-matter coupling and boosting the efficiency of solar energy conversion in photovoltaic devices. When plasmons decay, energetic electron-hole pairs are created through a non-radiative channel. These hot electrons have found applications in photodetection and photocatalysis but remain poorly understood in terms of mechanisms. In this work1, we made a comprehensive comparison between plasmon-induced hot carrier generation and direct excitations of hot carriers by photon absorption. Using a gold nanowire based hot carrier device, which either forms a Schottky barrier or an Ohmic barrier between nanostructures and a wide-bandgap semiconductor substrate, we are able to distinguish between these two mechanisms of hot carrier generation. We show that plasmon-induced hot electrons have higher energies than directly excited carriers, and can be characterized by the integration of electrical field enhancement within the nanostructures, while photoexcited carriers are correlated with material absorption. Our work paves the way for increasing the energy conversion efficiency by decreasing the Schottky barrier and collecting both the plasmonic and interband photocurrent, which may find wide applications in future photovoltaic devices.Item Absorption and scattering of single plasmonic nanoparticles(2010) Ha, Ji Won; Link, StephanIn this thesis, I present the absorption and scattering properties of single gold nanoparticles. I performed scanning electron microscopy (SEM) correlated dark-field scattering studies of gold nanorods (AuNRs). I found polarization-dependent scattering of a single AuNR. I studied the dependence of surface plasmon resonance (SPR) linewidth on both the refractive index of a surrounding medium and the adsorption of thiol groups onto the surface of AuNRs. I found that the SPR wavelength (lambdamax) shifts to longer wavelength when increasing the refractive index of the surrounding medium, while the SPR linewidth remains almost constant when increasing the refractive index of the surrounding environment. I also found that SPR wavelength shifts to longer wavelength as thiol groups bind to the surface of a single AuNR and that the SPR linewidth is broadened. I carried out photothermal imaging of gold nanospheres. The successful imaging of gold nanospheres down to 10 nm, which cannot be studied by conventional scattering-based methods, was achieved with a signal to noise (S/N) ratio of ∼35. I studied the size dependence of the photothermal signal of gold nanospheres with diameters ranging from 30 to 250 nm. The experimental results showed a very good agreement with Mie calculations for absorption of nanospheres. I further investigated polarization-dependent photothermal imaging of single AuNRs. It is observed that both gold nanowires and AuNRs show a polarization dependence in photothermal imaging. I utilized polarized photothermal imaging to determine the orientation of AuNRs. By selecting either the longitudinal or the transverse SPR mode, I precisely determined the orientation of individual AuNRs. Correlating SEM with photothermal images, the orientations of AuNRs were accurately measured. Most notably, I determined the orientation of an AuNR by exciting the transverse SPR mode which is not achievable by conventional scattering-based techniques.Item Absorption Localization in Plasmonic Heterostructures(2018-04-16) Hosseini Jebeli, Seyyed Ali; Link, StephanPlasmonic nanoparticles have found many applications in different areas such as nanoelectronics, energy harvesting and conversion, and photothermal therapy due to their strong interaction with light. The light absorption of homogenous plasmonic structures is mostly uniform and cannot be engineered easily. In this work, the absorption localization in heterogeneous structures is presented using different materials in the same structure as well as the same material in heterodimer structures composed of rods of different sizes. Pt decorated gold nanorods are found to have significant heat localization in the Pt particles due to their stronger light absorption than gold. The localization of absorption in the Pt is confirmed by performing correlated photoluminescence and scattering measurements as well as FDTD simulations. Additionally, gold nanorods of different sizes are coupled together to form a heterodimer. In this work, the results of changing the gap size between the nanorods are presented. This alters the amount of coupling between nanorods which allows control over the absorption intensity of each particle. Further, we find that the absorption hot spots can be switched in these structures by changing the excitation wavelength. Significant absorption localization is observed in both cases which demonstrates the great potential of plasmonic heterostructures for manipulating light-matter interactions.Item Absorption Spectroscopy of an Individual Fano Cluster(American Chemical Society, 2016) Yorulmaz, Mustafa; Hoggard, Anneli; Zhao, Hangqi; Wen, Fangfang; Chang, Wei-Shun; Halas, Naomi J.; Nordlander, Peter; Link, Stephan; Laboratory for NanophotonicsPlasmonic clusters can exhibit Fano resonances with unique and tunable asymmetric line shapes, which arise due to the coupling of bright and dark plasmon modes within each multiparticle structure. These structures are capable of generating remarkably large local electromagnetic field enhancements and should give rise to high hot carrier yields relative to other plasmonic nanostructures. While the scattering properties of individual plasmonic Fano resonances have been characterized extensively both experimentally and theoretically, their absorption properties, critical for hot carrier generation, have not yet been measured. Here, we utilize single-particle absorption spectroscopy based on photothermal imaging to distinguish between the radiative and nonradiative properties of an individual Fano cluster. In observing the absorption spectrum of individual Fano clusters, we directly verify the theoretical prediction that while Fano interference may be prominent in scattering, it is completely absent in absorption. Our results provide microscopic insight into the nature of Fano interference in systems of coupled plasmonic nanoparticles and should pave the way for the optimization of hot carrier production using plasmonic Fano clusters.Item Adsorption and Unfolding of a Single Protein Triggers Nanoparticle Aggregation(American Chemical Society, 2016) Dominguez-Medina, Sergio; Kisley, Lydia; Tauzin, Lawrence J.; Hoggard, Anneli; Shuang, Bo; Indrasekara, A. Swarnapali D.S.; Chen, Sishan; Wang, Lin-Yung; Derry, Paul J.; Liopo, Anton; Zubarev, Eugene R.; Landes, Christy F.; Link, StephanThe response of living systems to nanoparticles is thought to depend on the protein corona, which forms shortly after exposure to physiological fluids and which is linked to a wide array of pathophysiologies. A mechanistic understanding of the dynamic interaction between proteins and nanoparticles and thus the biological fate of nanoparticles and associated proteins is, however, often missing mainly due to the inadequacies in current ensemble experimental approaches. Through the application of a variety of single molecule and single particle spectroscopic techniques in combination with ensemble level characterization tools, we identified different interaction pathways between gold nanorods and bovine serum albumin depending on the protein concentration. Overall, we found that local changes in protein concentration influence everything from cancer cell uptake to nanoparticle stability and even protein secondary structure. We envision that our findings and methods will lead to strategies to control the associated pathophysiology of nanoparticle exposure in vivo.Item Adsorption of a Protein Monolayer via Hydrophobic Interactions Prevents Nanoparticle Aggregation under Harsh Environmental Conditions(American Chemical Society, 2013) Dominguez-Medina, Sergio; Blankenburg, Jan; Olson, Jana; Landes, Christy F.; Link, Stephan; Laboratory for NanophotonicsWe find that citrate-stabilized gold nanoparticles aggregate and precipitate in saline solutions below the NaCl concentration of many bodily fluids and blood plasma. Our experiments indicate that this is due to complexation of the citrate anions with Na+ cations in solution. A dramatically enhanced colloidal stability is achieved when bovine serum albumin is adsorbed to the gold nanoparticle surface, completely preventing nanoparticle aggregation under harsh environmental conditions where the NaCl concentration is well beyond the isotonic point. Furthermore, we explore the mechanism of the formation of this albumin "corona" and find that monolayer protein adsorption is most likely ruled by hydrophobic interactions. As for many nanotechnology-based biomedical and environmental applications, particle aggregation and sedimentation are undesirable and could substantially increase the risk of toxicological side-effects, the formation of the BSA corona presented here provides a low-cost bio-compatible strategy for nanoparticle stabilization and transport in highly ionic environments.Item Angle- and Spectral-Dependent Light Scattering from Plasmonic Nanocups(2013-06-05) Li, Yang; Nordlander, Peter J.; Halas, Naomi J.; Link, StephanThe interaction of light with small designed particles and structures gives rise to an increasing number of phenomena of potentially dramatic technological importance, such as metamaterials, superlens focusing, and enhanced spectroscopy. Metallic nanostructures with their geometry-dependent optical resonances are a topic of intense current interest due to their ability to manipulate light in ways not possible with conventional optical materials. A particularly fascinating aspect of these systems is the recently realized possibility of creating optical frequency “magnetic plasmon” responses of comparable magnitude to the “electric plasmon” response. Au nanocups at their magnetoinductive resonance have the unique ability to redirect scattered light in a direction dependent on cup orientation, as a true three-dimensional nanoantenna. As optical frequency nanoantennas, reduced-symmetry plasmonic nanoparticles have light-scattering properties that depend strongly on geometry, orientation, and variations in dielectric environment. Here we investigate how these factors influence the spectral and angular dependence of light scattered by Au nanocups. A simple dielectric substrate causes the axial, electric dipole mode of the nanocup to deviate substantially from its characteristic cos square free space scattering profile, while the transverse, magnetic dipole mode remains remarkably insensitive to the presence of the substrate. Nanoscale irregularities of the nanocup rim and the local substrate permittivity have a surprisingly large effect on the spectral- and angle-dependent light-scattering properties of these structures. The different angular scattering and wavelength response from the axial and transverse nanocup modes make the nanocup an interesting particle for the nanoscale manipulation of light in three dimensions. The sensitivity of this system to geometric and environmental factors may present opportunities for active, substrate-mediated control of light scattering.Item Bleach Imaged Plasmon Propagation (BIIPP) in Single Gold Nanowires(2011) Solis, David Enrique, Jr.; Link, StephanHere, we present a novel approach to visualize propagating surface plasmon polaritons through plasmon-exciton interactions between single gold nanowires and a thin film of a fluorescent polymer. A plasmon polariton was launched by exciting one end of a single gold nanowire with a 532 nm laser. The local near-field of the propagating plasmon modes caused bleaching of the polymer emission. The degree of photobleaching along the nanowire could be correlated with the propagation distance of the surface plasmon polaritons. Using this method of bleach-imaged plasmon propagation (BIIPP), we determined a plasmon propagation distance of 1.8 ± 0.4 μm at 532 nm for chemically grown gold nanowires. Our results are supported by finite difference time domain electromagnetic simulations.Item Bleach Imaged Plasmon Propagation (BlIPP) of Metallic Nanoparticle Waveguides(2013-09-16) Solis, David; Link, Stephan; Landes, Christy F.; Johnson, Bruce R.The high speed transfer of information in materials with dimensions below the sub-diffraction limit is essential for future technological developments. Metallic nanoparticle (NP) waveguides serve a unique role in efficient energy transfer in this size regime. Light may be confined to metallic structures and propagate along the surface of the waveguide via propagating plasmon waves known as surface plasmon polaritons (SPPs). Plasmon propagation of energy in metallic structures is not perfect however and damping losses from the waveguide material lead to a characteristic exponential decay in the plasmon near field intensity. This decay length is known as the propagation length and serves as an excellent metric to compare various waveguide materials and structures to one another at particular excitation wavelengths. This thesis presents recent work in the development of a novel measurement technique termed bleach imaged plasmon propagation (BlIPP). BlIPP uses the photobleaching property of fluorophores and far field fluorescence microscopy to probe the near-field intensity of propagating plasmons and determine the propagation length. The experimental setup, image analysis, conditions, and application of BlIPP are developed within this thesis and an in depth review of the 1-photon photobleaching mechanism is also investigated. The BlIPP method is used to investigate long plasmon propagation lengths along straight chains of tightly packed Au NPs through the coupling of light to sub-radiant propagating modes, where radiative energy losses are suppressed. The findings of this work reveal, experimentally, the importance of small gap distances for the propagation of energy. Complex chain architectures are then explored using BlIPP measurements of tightly packed straight and bent chains of spherical silver NPs. We observe the highly efficient propagation of energy around sharp corners with no additional bending losses. The findings of this thesis demonstrate the advantages and capabilities of using BlIPP propagation length measurement. Further, BlIPP is used to reveal the advantage of coupling light to sub-radiant modes of NP chains, which demonstrate the ability to guide light efficiently across long distances and around complex structures, bringing us a step closer to the goal of applying plasmonic devices and circuitry in ultra compact opto-electronic devices.Item Chemical Interface Damping of Surface Plasmon Resonances(American Chemical Society, 2021) Lee, Stephen A.; Link, Stephan; Smalley Curl InstituteConspectusMetal nanoparticles have been utilized for a vast amount of plasmon enhanced spectroscopies and energy conversion devices. Their unique optical properties allow them to be used across the UV–vis-NIR spectrum tuned by their size, shape, and material. In addition to utility in enhanced spectroscopy and energy/charge transfer, the plasmon resonance of metal nanoparticles is sensitive to its surrounding environment in several ways. The local refractive index determines the resonance wavelength, but plasmon damping, as indicated by the homogeneous line width, also depends on the surface properties of the metal nanoparticles. Plasmon oscillations can decay through interband, intraband, radiation, and surface damping. While the first three damping mechanisms can be modeled based on bulk dielectric data using electromagnetic simulations, surface damping does not depend on the material properties of the nanoparticle alone but rather on the interface composition between the nanoparticle and its surrounding environment. In this Account, we will discuss three different metal nanoparticle interfaces, identifying the surface damping contribution from chemical interface damping and how it manifests itself in different interface types. On the way to uncovering the various damping contributions, we use three different single-particle spectroscopic techniques that are essential to measuring homogeneous plasmon line widths: darkfield scattering, photothermal heterodyne imaging, and photoluminescence microscopies. Obtaining the homogeneous plasmon spectrum through single-particle spectroscopy is paramount to measuring changes in plasmon damping, where even minor size and shape heterogeneities can completely obfuscate the broadening caused by surface damping. Using darkfield scattering spectroscopy, we first describe a model for chemical interface damping by expanding upon the surface damping contribution to the plasmon resonance line width to include additional influences due to adsorbed molecules. Based on the understanding of chemical interface damping as a surface damping mechanism, we then carefully compare how two molecular isomers lead to greatly different damping rates upon adsorption to gold nanorods due to differences in the formation of image dipoles within the metal nanoparticles. This plasmon damping dependence on the chemical identity of the interface is strongly correlated with the chemical’s electronegativity. A similar damping trend is observed for metal oxide semiconductors, where the metal oxide with greater electron affinity leads to larger interface damping. However, in this case, the mechanism is different for the metal oxide interfaces, as damping occurs through charge transfer into interfacial states. Finally, the damping effect of catalytic metal nanoislands on gold nanorods is compared for the three spectroscopic methods mentioned. Through correlated single-particle scattering, absorption, and photoluminescence spectroscopy, the mechanism for metal–metal interface damping is determined most likely to arise from an enhanced absorption, although charge transfer cannot be ruled out. From this body of research, we conclude that chemical interface damping is a major component of the total damping rate of the plasmon resonance and critically depends on the chemical interface of the metallic nanoparticles. Plasmon damping occurs through distinct mechanisms that are important to differentiate when considering the purpose of the plasmonic nanoparticle: enhanced spectroscopy, energy conversion, or catalysis. It must also be noted that many of the mechanisms are currently indifferentiable, and thus, new single-particle spectroscopic methods are needed to further characterize the mechanisms underlying chemical interface damping.Item Chiral and Achiral Nanodumbbell Dimers: The Effect of Geometry on Plasmonic Properties(American Chemical Society, 2016) Smith, Kyle W.; Zhao, Hangqi; Zhang, Hui; Sánchez -Iglesias, Ana; Grzelczak, Marek; Wang, Yumin; Chang, Wei-Shun; Nordlander, Peter; Liz-Marzán, Luis; Link, Stephan; Laboratory for NanophotonicsMetal nanoparticles with a dumbbell-like geometry have plasmonic properties similar to those of their nanorod counterparts, but the unique steric constraints induced by their enlarged tips result in distinct geometries when self-assembled. Here, we investigate gold dumbbells that are assembled into dimers within polymeric micelles. A single-particle approach with correlated scanning electron microscopy and dark-field scattering spectroscopy reveals the effects of dimer geometry variation on the scattering properties. The dimers are prepared using exclusively achiral reagents, and the resulting dimer solution produces no detectable ensemble circular dichroism response. However, single-particle circular differential scattering measurements uncover that this dimer sample is a racemic mixture of individual nanostructures with significant positive and negative chiroptical signals. These measurements are complemented with detailed simulations that confirm the influence of various symmetry elements on the overall peak resonance energy, spectral line shape, and circular differential scattering response. This work expands the current understanding of the influence self-assembled geometries have on plasmonic properties, particularly with regard to chiral and/or racemic samples which may have significant optical activity that may be overlooked when using exclusively ensemble characterization techniques.Item Chiral Plasmonic Pinwheels Exhibit Orientation-Independent Linear Differential Scattering under Asymmetric Illumination(American Chemical Society, 2023) McCarthy, Lauren A.; Verma, Ojasvi; Naidu, Gopal Narmada; Bursi, Luca; Alabastri, Alessandro; Nordlander, Peter; Link, StephanPlasmonic nanoantennas have considerably stronger polarization-dependent optical properties than their molecular counterparts, inspiring photonic platforms for enhancing molecular dichroism and providing fundamental insight into light-matter interactions. One such insight is that even achiral nanoparticles can yield strong optical activity when they are asymmetrically illuminated from a single oblique angle instead of evenly illuminated. This effect, called extrinsic chirality, results from the overall chirality of the experimental geometry and strongly depends on the orientation of the incident light. Although extrinsic chirality has been well-characterized, an analogous effect involving linear polarization sensitivity has not yet been discussed. In this study, we investigate the differential scattering of rotationally symmetric chiral plasmonic pinwheels when asymmetrically irradiated with linearly polarized light. Despite their high rotational symmetry, we observe substantial linear differential scattering that is maintained over all pinwheel orientations. We demonstrate that this orientation-independent linear differential scattering arises from the broken mirror and rotational symmetries of our overall experimental geometry. Our results underscore the necessity of considering both the rotational symmetry of the nanoantenna and the experimental setup, including illumination direction and angle, when performing plasmon-enhanced chiroptical characterizations. Our results demonstrate spectroscopic signatures of an effect analogous to extrinsic chirality for linear polarizations.Item Chiral plasmonics of self-assembled nanorod dimers(Nature Publishing Group, 2013) Ma, Wei; Kuang, Hua; Wang, Libing; Xu, Liguang; Chang, Wei-Shun; Zhang, Huanan; Sun, Maozhong; Zhu, Yinyue; Zhao, Yuan; Liu, Liqiang; Xu, Chuanlai; Link, Stephan; Kotov, Nicholas A.Chiral nanoscale photonic systems typically follow either tetrahedral or helical geometries that require four or more different constituent nanoparticles. Smaller number of particles and different chiral geometries taking advantage of the self-organization capabilities of nanomaterials will advance understanding of chiral plasmonic effects, facilitate development of their theory, and stimulate practical applications of chiroplasmonics. Here we show that gold nanorods self-assemble into side-by-side orientated pairs and ‘‘ladders’’ in which chiral properties originate from the small dihedral angle between them. Spontaneous twisting of one nanorod versus the other one breaks the centrosymmetric nature of the parallel assemblies. Two possible enantiomeric conformations with positive and negative dihedral angles were obtained with different assembly triggers. The chiral nature of the angled nanorod pairs was confirmed by 4p full space simulations and the first example of single-particle CD spectroscopy. Self-assembled nanorod pairs and ‘‘ladders’’ enable the development of chiral metamaterials, (bio)sensors, and new catalytic processes.Item Chiral templating of self-assembling nanostructures by circularly polarized light(Nature Publishing Group, 2015) Yeom, Jihyeon; Yeom, Bongjun; Chan, Henry; Smith, Kyle W.; Dominguez-Medina, Sergio; Bahng, Joong Hwan; Zhao, Gongpu; Chang, Wei-Shun; Chang, Sung Jin; Chuvilin, Andrey; Melnikau, Dzmitry; Rogach, Andrey L.; Zhang, Peijun; Link, Stephan; Král, Petr; Kotov, Nicholas A.The high optical and chemical activity of nanoparticles (NPs) signifies the possibility of converting the spin angular momenta of photons into structural changes in matter. Here, we demonstrate that illumination of dispersions of racemic CdTe NPs with right- (left-)handed circularly polarized light (CPL) induces the formation of right- (left-)handed twisted nanoribbons with an enantiomeric excess exceeding 30%, which is ∼10 times higher than that of typical CPL-induced reactions. Linearly polarized light or dark conditions led instead to straight nanoribbons. CPL 'templating' of NP assemblies is based on the enantio-selective photoactivation of chiral NPs and clusters, followed by their photooxidation and self-assembly into nanoribbons with specific helicity as a result of chirality-sensitive interactions between the NPs. The ability of NPs to retain the polarization information of incident photons should open pathways for the synthesis of chiral photonic materials and allow a better understanding of the origins of biomolecular homochirality.Item Circular Differential Scattering of Single Chiral Self-Assembled Gold Nanorod Dimers(American Chemical Society, 2015) Wang, Lin-Yung; Smith, Kyle W.; Dominiquez-Medina, Sergio; Moody, Nicole; Olson, Jana M.; Zhang, Huanan; Chang, Wei-Shun; Kotov, Nicholas; Link, Stephan; Laboratory for NanophotonicsCircular dichroism spectroscopy is essential for structural characterization of proteins and chiral nanomaterials. Chiral structures from plasmonic materials have extraordinary strong circular dichroism effects compared to their molecular counterparts. While being extensively investigated, the comprehensive account of circular dichroism effects consistent with other plasmonic phenomena is still missing. Here we investigated the circular differential scattering of a simple chiral plasmonic system, a twisted side-by-side Au nanorod dimer, using single-particle circular dichroism spectroscopy complimented with electromagnetic simulations. This approach enabled us to quantify the effects of structural symmetry breaking, namely, size-mismatch between the constituent Au nanorods and large twist angles on the resulting circular differential scattering spectrum. Our results demonstrate that, if only scattering is considered as measured by dark-field spectroscopy, a homodimer of Au nanorods with similar sizes produces a circular differential scattering line shape that is different from the bisignate response of the corresponding conventional CD spectrum, which measures extinction, that is, the sum of scattering and absorption. On the other hand, symmetry breaking in a heterodimer with Au nanorods with different sizes yields a bisignate circular differential scattering line shape. In addition, we provide a general method for correcting linear dichroism artifacts arising from slightly elliptically polarized light in a typical dark-field microscope, as is necessary especially when measuring highly anisotropic nanostructures, such as side-by-side nanorods. This work lays the foundation for understanding absorption and scattering contributions to the CD line shape of single chiroplasmonic nanostructures free from ensemble-averaging, especially important for self-assembled chiral nanostructures that usually exist as both enantiomers.Item Development and Application of Methods to Study Nanoparticle Diffusion Using Intensity Correlation Spectroscopy(2011) Tcherniak, Alexei; Link, StephanThe practical application of nanoparticles requires transitioning from well controlled experimental settings to highly variable "real-life" conditions. Understanding the resulting changes in the behavior and stability of nanoparticles is therefore of paramount importance. This thesis discusses the development and practical applications of tools to monitor the behavior of nanoparticles in real-time using intensity correlation spectroscopy techniques. I show how-correlation spectroscopy can be adapted to nanoparticle systems; and provide particular parameters and settings especially vital for heterogeneous systems. Oftentimes nanoparticles have to be labeled to be detected, which can complicate the system of study and can introduce systematic errors into the analysis. Intensity correlation spectroscopy was tested on dye-labeled magnetite nanocrystals. The fluorescence correlation spectroscopy results were surprisingly biased towards a low concentration of aggregates. Scattering and absorption cross-sections of gold nanoparticles are greatly enhanced near the plasmon resonance wavelength, providing strong intrinsic signals for directly visualizing nanoparticles. I show here how scattering and absorption scale with nanoparticle size; and how size heterogeneity within nanoparticle samples translates into the detected signals. One-photon luminescence of gold nanoparticles, an often neglected signal, was also considered. A comparison between one-photon luminescence and scattering correlation spectroscopy revealed that the former has a much smaller bias towards aggregates and therefore is advantageous in systems prone to aggregation. Overall, the work presented here describes the tools and methods that were developed towards better understanding of nanoparticle behavior in a liquid medium where they are to be employed for environmental and biological applications.Item Development of an Active Display Using Plasmonic Nanorods and Liquid Crystals(2015-08-26) Olson, Jana; Link, Stephan; Halas, Naomi; Nordlander, PeterThe current generation of display technology has already provided the public with displays that exhibit high color fidelity, faster than the eye can see frame rates, and screens spanning an order of magnitude in size, from hand-held to wall-sized. What has yet to be achieved is for display technology to become versatile, fitting into the human environment in a way that is unobtrusive, ergonomic, and promotes an improved quality of life. This thesis points to nanotechnology, particularly plasmonic nanomaterials, as a way to bring display technologies to that next level. The work presented in this thesis is the development of a nanomaterial that is vividly colored, electronically controllable, and highly versatile in its possible applications. First, an understanding of the electronic switching of nematic liquid crystals is gained via combination of randomly oriented gold nanorods with homogeneously aligned nematic liquid crystal. The nanorods are used to probe the birefringence of the liquid crystal as an in-plane bias is used to switch the alignment of the liquid crystal from a uniform parallel alignment to a 90 degree twisted alignment. This mechanism is confirmed with theoretical modeling using Jones Calculus. Second, the polarization sensitivity of the nanorods is exploited by creating a hexagonal array of co-oriented nanorods to form a plasmonic pixel using electron beam lithography. Aluminum was chosen as the plasmonic material because its plasmon resonance can span the whole visible region, and because of its compatibility with the semiconductor manufacturing industry. The outstanding vivid color of these pixels is dependent on the physical characteristics of the individual nanorods, and also on the inter-rod spacing. Dipole coupling within the array of ~300 nm separated nanorods is used to restrict plasmon scattering at both long- and short-wavelengths. A simple 3-step color control mechanism was developed that others can use to produce aluminum plasmonic pixels with colors on-par with standard red, green, and blue displays. Finally, mm-scale colored pixels are demonstrated to be switchable with a liquid crystal display, and therefore immediately compatible with current liquid crystal display technology.Item Dye-Assisted Gain of Strongly Confined Surface Plasmon Polaritons in Silver Nanowires(American Chemical Society, 2014) Paul, Aniruddha; Zhen, Yu-Rong; Wang, Yi; Chang, Wei-Shun; Xia, Younan; Nordlander, Peter; Link, Stephan; Laboratory for NanophotonicsSubwavelength confinement and active control of light is essential for nanoscale communication devices at visible frequencies that support large bandwidths.[1-5] Noble-metal nanostructures present an excellent platform for strongly confined optical waveguides [6-13] because of their ability to support surface plasmon polaritons (SPPs).[14] However, SPP propagation suffers from losses that seriously limit their application potential. [9] Although significant progress toward SPP loss compensation has been reported for various planar 2D waveguide structures,[15-20] as well as lasing involving strongly localized plasmon modes,[21,22] SPP gain in 1D nanoscale waveguides at visible frequencies is yet to be accomplished. Here, we report the first demonstration of gain for deep subwavelength confined SPPs (mode area = λ2/40) in chemically prepared silver nanowires (Ag NWs). We measured a gain coefficient of 270 cm-1 resulting in 14% loss compensation using a continuous-wave (cw) pump laser. These results are an important step toward total loss compensation for highly confined nanowire SPPs.Item Effects of Geometry on Plasmonic Properties of Nanostructures(2018-04-12) Smith, Kyle Warren; Link, StephanThe role of polarization in light-matter interactions is a subject of study as old as optics itself and is critical to numerous optical devices. Plasmonic metal nanoparticles, which are potential candidates for many applications due to their unique optical properties, have sensitive polarization responses determined by their nanoscale geometry and local environment. Polarization resolved studies of individual metal nanoparticles of different shapes and geometries are reported with an emphasis on the role of chirality. A novel microscopy method for monitoring chirality in single nanoparticles using differential scattering of circular polarizations is reported, complete with a correction method for linear polarization artifacts. This method is demonstrated through the measurement of self-assembled, twisted gold nanorod dimers. The same method is used to characterize racemically assembled dimers of gold dumbbell shaped particles where individual dimers were shown to have chiral responses, despite no ensemble chiral response. Triangular bifrustrum shaped gold nanoparticles are shown to have their scattering response split into non-degenerate, orthogonal, linearly polarized modes as a direct result of them sitting tilted on a substrate. Lithographically prepared curved gold nanostructures are shown to have very strong scattering modulation in response to switching the handedness of incident circular polarizations, yet a detailed investigation shows this modulation is not driven by helical field oscillations as in traditional circular dichroism. Instead planar, trochoid-type field oscillations present in evanescent fields are shown to be responsible for the dramatic response, which opens the door to an entirely new form of dichroism. In summary, we show that metal nanostructures offer a rich platform to study, manipulate, and utilize light polarization.Item Electron-phonon relaxation dynamics of hot electrons in gold nanoparticles(2023-10-26) Bruncz, Autumn Rose; Link, StephanWe report on a pump wavelength-dependent study of the electron-phonon relaxation in colloidal solutions of isolated gold nanospheres and nanorods. We varied the pump wavelength from 400 to 800 nm to cover interband, plasmon, and intraband excitations in 40 nm gold nanospheres with a plasmon resonance at ~525 nm, as well as in 34×91 nm gold nanorods with longitudinal plasmon resonance at 720 nm. We also performed an excitation fluence dependence study at each pump wavelength and plotted the measured plasmon bleach recovery kinetics against change in electronic temperature. These plots were all linear and revealed as the intercept of zero excitation power the intrinsic electron-phonon relaxation time, which we found to be independent of pump wavelength and hence the same regardless of how the nanoparticles were excited. Similarly, the slopes of the excitation power dependencies varied remained constant, from taking into account the absorption spectra of each nanoparticle sample. The plasmon bleach recovery dynamics at a given excitation fluence simply describe the collective cooling of a hot thermalized electron distribution with the lattice to reach thermal equilibrium and are longer the larger the initial temperature rise.